UniProtKB description: Catalytic subunit of the vacuolar transporter chaperone (VTC) complex. The VTC complex acts as vacuolar polyphosphate polymerase that catalyzes the synthesis of inorganic polyphosphate (polyP) via transfer of phosphate from ATP to a growing polyP chain, releasing ADP. VTC exposes its catalytic domain VTC4 to the cytosol, where the growing polyP chain winds through a tunnel-shaped pocket, integrating cytoplasmic polymer synthesis with polyP membrane translocation (PubMed:19390046). The VTC complex carries 9 vacuolar transmembrane domains, which are likely to constitute the translocation channel into the organelle lumen (PubMed:25315834, PubMed:19390046). PolyP synthesis is tightly coupled to its transport into the vacuole lumen, in order to avoid otherwise toxic intermediates in the cytosol, and it depends on the proton gradient across the membrane, formed by V-ATPase (PubMed:25315834). The VTC complex also plays a role in vacuolar membrane fusion (PubMed:11102525, PubMed:11823419, PubMed:12584253). Required for SEC18/NSF activity in SNARE priming, membrane binding of LMA1 and V(0) trans-complex formation (PubMed:11823419). Binds inositol hexakisphosphate (Ins6P) and similar inositol polyphosphates, such as 5-diphospho-inositol pentakisphosphate (5-InsP7); these are important intracellular signaling molecules. Inositol polyphosphate binding promotes vacuolar polyphosphate synthesis (PubMed:27080106). The VTC complex is required for microautophagy. It is a constituent of autophagic tubes and is required for scission of microautophagic vesicles from these tubes (PubMed:17079729).